.TH std::atomic_fetch_sub,std::atomic_fetch_sub_explicit 3 "2024.06.10" "http://cppreference.com" "C++ Standard Libary"
.SH NAME
std::atomic_fetch_sub,std::atomic_fetch_sub_explicit \- std::atomic_fetch_sub,std::atomic_fetch_sub_explicit

.SH Synopsis
   Defined in header <atomic>
   template< class T >

   T atomic_fetch_sub( std::atomic<T>* obj,                           \fB(1)\fP \fI(since C++11)\fP

                       typename std::atomic<T>::difference_type arg )
   noexcept;
   template< class T >

   T atomic_fetch_sub( volatile std::atomic<T>* obj,                  \fB(2)\fP \fI(since C++11)\fP

                       typename std::atomic<T>::difference_type arg )
   noexcept;
   template< class T >

   T atomic_fetch_sub_explicit( std::atomic<T>* obj,
                                typename                              \fB(3)\fP \fI(since C++11)\fP
   std::atomic<T>::difference_type arg,

                                std::memory_order order ) noexcept;
   template< class T >

   T atomic_fetch_sub_explicit( volatile std::atomic<T>* obj,
                                typename                              \fB(4)\fP \fI(since C++11)\fP
   std::atomic<T>::difference_type arg,

                                std::memory_order order ) noexcept;

   Performs atomic subtraction. Atomically subtracts arg from the value pointed to by
   obj and returns the value obj held previously. The operation is performed as if the
   following was executed:

   1,2) obj->fetch_sub(arg)
   3,4) obj->fetch_sub(arg, order)

   If std::atomic<T> has no fetch_sub member (this member is only provided for integral
   , floating-point
   \fI(since C++20)\fP and pointer types except bool), the program is ill-formed.

.SH Parameters

   obj   - pointer to the atomic object to modify
   arg   - the value to subtract from the value stored in the atomic object
   order - the memory synchronization ordering

.SH Return value

   The value immediately preceding the effects of this function in the modification
   order of *obj.

.SH Example

   Multiple threads may use std::atomic_fetch_sub to concurrently process an indexed
   container.


// Run this code

 #include <atomic>
 #include <iostream>
 #include <numeric>
 #include <string>
 #include <thread>
 #include <vector>

 const int N = 50;
 std::atomic<int> cnt;
 std::vector<int> data(N);

 void reader(int id)
 {
     for (;;)
     {
         int idx = atomic_fetch_sub_explicit(&cnt, 1, std::memory_order_relaxed);
         if (idx >= 0)
             std::cout << "reader " << std::to_string(id) << " processed item "
                       << std::to_string(data[idx]) << '\\n';
         else
         {
             std::cout << "reader " << std::to_string(id) << " done\\n";
             break;
         }
     }
 }

 int main()
 {
     std::iota(data.begin(), data.end(), 1);
     cnt = data.size() - 1;

     std::vector<std::thread> v;
     for (int n = 0; n < 5; ++n)
         v.emplace_back(reader, n);
     for (auto& t : v)
         t.join();
 }

.SH Output:

 reader 2 processed item 50
 reader 1 processed item 44
 reader 4 processed item 46
 <....>
 reader 0 done
 reader 4 done
 reader 3 done

   Defect reports

   The following behavior-changing defect reports were applied retroactively to
   previously published C++ standards.

     DR    Applied to         Behavior as published         Correct behavior
   P0558R1 C++11      exact type match was required because T is only deduced
                      T was deduced from multiple arguments from obj

.SH See also

                             atomically subtracts the argument from the value stored in
   fetch_sub                 the atomic object and obtains the value held previously
                             \fI(public member function of std::atomic<T>)\fP
   atomic_fetch_add          adds a non-atomic value to an atomic object and obtains
   atomic_fetch_add_explicit the previous value of the atomic
   \fI(C++11)\fP                   \fI(function template)\fP
   \fI(C++11)\fP
   C documentation for
   atomic_fetch_sub,
   atomic_fetch_sub_explicit
